Fluid mixtures can be separated by selective diffusion through membranes under concentration or pressure gradients by utilizing differences in transport and thermodynamic partition or equilibrium properties of the mixture components in the membrane materials. One widely-used type of membrane comprises a non-porous polymer in which the mixture components selectively dissolve and selectively permeate or diffuse in the soluble state through the polymer to yield a permeate product enriched in the selectively diffusing components and a non-permeate or reject product enriched in the remaining components. A second type of membrane comprises a porous solid in which the mixture components selectively diffuse or permeate in the fluid state through the pores to yield a permeate product enriched in the selectively diffusing components and a non-permeate or reject product enriched in the remaining components.
There are four mechanisms by which fluid mixtures, in particular gas mixtures, can be separated by a solid porous membrane. The first of these is diffusion in the gas phase through pores having diameters approaching the mean free path dimensions of the molecules in the gas mixture, which is often termed Knudsen flow or Knudsen diffusion. These pores are small enough, however, to preclude bulk gas flow by molecular diffusion. By this Knudsen flow mechanism, gas molecules of differing molecular weights collide with the pore walls, thus yielding a selective separation in which the permeation rate of each component is inversely proportional to the square root of its molecular weight. The phenomenon of gas diffusion and separation by Knudsen flow through porous solids is well known, and is described in standard textbooks such as "Mass Transfer in Heterogeneous Catalysis", by C. N. Satterfield, MIT Press, 1969.
A second type of separation mechanism for the separation of gas mixtures by porous solids occurs when the diameters of the pores are larger than the largest molecular diameter of the components in the gas mixture and range up to about 40-100 Angstroms in diameter. At the appropriate temperature and pressure conditions, certain components in the gas mixture will condense within the pores by capillary or Kelvin condensation and flow through the pores as a condensed phase under a capillary pressure gradient across the membrane. Condensed molecules within the pores hinder or eliminate the diffusion of non-condensing molecules, and a selective separation between components in the gas mixture is thus accomplished.
A third type of separation mechanism occurs when the pore diameters of the membrane are larger than the largest molecular diameter of the components in the gas mixture and typically smaller than about 2 to 5 times this diameter, and thus are smaller than pores in which Knudsen diffusion dominates. These pores have typical diameters of about 3 to 20 Angstroms and are termed micropores by the classification definition of the International Union of Pure and Applied Chemistry (I.U.P.A.C.). In the present disclosure, the term "pores" will be used to denote pores of any size, including micropores. When a gas mixture is contacted with a porous membrane at temperature and pressure conditions well-removed from conditions at which capillary condensation occurs, the separation mechanism defined as selective surface flow or selective surface diffusion can occur under a pressure gradient across the membrane. This mechanism is characterized by the selective adsorption of certain mixture component molecules within the pores and the surface flow of these molecules in the adsorbed phase through the pores. Furthermore, the adsorbed phase hinders the gas-phase Knudsen diffusion of non-adsorbed or weakly adsorbed component molecules through the pores, and an enhanced selective separation between components in the gas mixture is thus accomplished.
The fourth mechanism by which gas mixtures are separated by a solid porous membrane material is that of molecular sieving in which essentially all of the pores are larger than certain component molecules and smaller than other component molecules in the mixture. Larger molecules cannot enter these pores, while smaller molecules can enter and diffuse through the pores, and a selective separation based upon exclusion by molecular size is thus accomplished.
Since porous solids contain a distribution of pore sizes, more than one of these mechanisms can occur simultaneously depending upon the actual pore size distribution and sizes of component molecules in the gas mixture, as well as the pressure and temperature. However, a single mechanism usually dominates and the resulting mixture separation is essentially accomplished by means of that dominant mechanism.
The separation of gaseous mixtures by capillary condensation in solid porous membranes is described by M. Asaeda and L. D. Du in a paper entitled "Separation of Alcohol/Water Gaseous Mixtures by a Thin Ceramic Membrane" in the Journal of Chemical Engineering of Japan, Vol. 19, No. 1, pp. 72-77 (1986). This article describes the preparation of a silica-alumina membrane about 10 microns thick, having pores ranging between 2 and 10 nanometers (20 to 100 Angstroms) in diameter, which is supported on a porous ceramic substrate. The silica-alumina membrane was prepared by applying alumina sol to the substrate, drying at 80.degree. C., and firing at 450.degree. C. The pores were decreased to the desired size by treatment with aluminum isopropoxide and with dilute sodium silicate solution, and the resulting membrane was aged in humid air and washed in boiling water. The resulting membrane was contacted in a series of experiments with binary saturated gaseous mixtures of water and methanol, water and ethanol, and water and isopropanol. Water condensed in the pores of the membrane by capillary or Kelvin condensation, and the permeability of water in all cases was higher than the permeabilities of the alcohols, all of which have a higher molecular size and weight than water. The water/alcohol gas mixtures were separated by the mechanisms of capillary condensation and molecular sieving.
U.S. Pat. No. 4,583,996 discloses an apparatus comprising an inorganic hydrophilic porous membrane for separating a condensible gas, especially water vapor, from a gas mixture at conditions under which the condensible gas selectively permeates through the membrane to the exclusion of non-condensing gases. An inorganic membrane of sodium borosilicate glass is disclosed for such a separation, in which the pores range up to 100 Angstroms in diameter and preferably are from 4 to 40 Angstroms in diameter, and which has a preferred thickness of 5 microns to 1 mm. Other condensible vapors are disclosed which can be recovered by these membranes such as acetic acid, toluene, and n-propylamine.
U.S. Pat. No. 3,511,031 also discloses an aparatus for removing water vapor from a gas by means of capillary condensation by hydrophilic membranes having mean pore diameters between 3 and 100 Angstroms. These membranes were fabricated in the form of plates or tubes made of porous carbon or porous glass having a thickness of about 1.2 mm.
U.S. Pat. No. 3,022,187 discloses a porous membrane for separation of a gas mixture by gaseous diffusion which is made by depositing particles of a metallic oxide within the pores of a sintered porous metallic support. The metallic oxide particles, having dimesions on the order of 100 to 10,000 Angstroms, are suspended in a fluid and the resulting suspension is drawn into the pores of the metallic support to deposit the particles therein. The membrane is then dried, yielding pores with an average mean diameter of 120 Angstroms as determined by permeability measurements.
The preparation and use of activated carbon membranes for gas separation by molecular sieving are described in an article by J. E. Koresh and A. Sofer entitled "Molecular Sieve Carbon Perm-selective Membrane, Part 1, Presentation of a New Device for Gas Mixture Separation", in Separation Science and Technology 18(8), pp. 723-734 (1983). Activated carbon membranes in the form of hollow fibers were prepared by pyrolysis of polymeric hollow fibers at 800.degree. C. and 950.degree. C. to produce porous hollow fiber membranes with various pore size distributions and a wall thickness of about 6 microns. Pore diameters were shown to range from those which allow binary mixture separations by molecular sieving a size exclusion up to larger diameters which allow Knudsen flow. Pure-component permeability of these various membranes were measured for He, O.sub.2, N.sub.2, and SF.sub.6.
U.S. Pat. No. 4,685,940 discloses porous carbon membranes for separating gas mixtures by molecular sieving. One group of membranes was prepared by pyrolysis of cellulose hollow fiber or flat sheet membranes at 950.degree. C. to yield a porous membrane with a pore size range of 2-2.5 Angstroms with a very sharp cutoff above 2.5 Angstroms as determined by low permeability to helium and hydrogen and undetectable permeability to nitrogen, methane, and sulfur hexafluoride. A similar membrane heated only to 800.degree. C. yielded a pore size range of 2.5 to 3.0 Angstroms with a sharp cutoff above 3.0 Angstroms as determined by high permeability to helium and hydrogen and negligible permeability to nitrogen and methane. Other membranes having similar properties were prepared by pyrolysis of a commercial asymmetric polymeric membrane, by chemical vapor deposition of various organic gases on a porous graphite tube, or by plasma deposition of carbon on porous graphite. Post treatment of selected membranes by various methods to enlarge the pores was carried out to provide for molecular sieving of larger molecules. These membranes, which have pore sizes generally in the range of 2.5 to 5.0 Angstroms, are specifically prepared for separating binary gas mixtures in which the molecular sizes differ by 10% or less. The specific pore size for a given separation is such that the pore diameters are between the molecular diameters of the two gases to be separated, and there is no significant number of pores which are 10% or larger in size than the smaller of the molecular sizes of the two gases.
U.S. Pat. No. 4,699,892 describes asymmetric composite membranes which have an ultrathin layer of zeolite on a porous substrate and methods for the production of such membranes. The active ultrathin layer is a cage-shaped zeolite composed of a 6-, 8-, 10-, or 12-membered oxygen ring window having aluminum or gallium atoms, silicon or germanium atoms, and oxygen atoms as constituent members. These zeolite crystals have pore sizes in the range of about 3 to 12 Angstroms which are useful for separating gas mixtures by molecular sieving and which also may have catalytic activity. The use of such membranes is disclosed for the separation by molecular sieving of n-paraffins and n-olefins from light petroleum fractions; butene-1 from C.sub.4 hydrocarbon fractions; H.sub.2 S, CO.sub.2, and mercaptans from LPG; H.sub.2 O, CO.sub.2, N.sub.2, and hydrocarbons from air to yield oxygen-enriched air; and the separation of argon from oxygen.
An apparatus for removing water vapor from gases by capillary condensation using a porous hydrophilic membrane is disclosed in U.S. Pat. No. 3,511,031. The membrane used is homogeneous, has a mean pore diameter between 3 and 100 Angstroms, and is made of porous glass or porous pressed carbon.
The diffusion of adsorbable gas mixtures through porous media by the mechanism of surface flow was described in a paper by R. Ash, R. M. Barrer, F. R. S. Pope, and C. G. Pope entitled "Flow of Adsorbable Gases and Vapors in a Microporous Medium. II. Binary Mixtures" in Proceedings of the Royal Society A271, 19, pp. 19-33 (1963). Several binary gas mixtures were fractionated by diffusion through a high surface area porous carbon plug 0.91 cm in length. The carbon plug was made by compacting non-porous, micron-sized graphitized carbon black particles which provided a high surface area carbon plug with microporosity between the carbon particles. The dominant mode of diffusion of the less volatile, more strongly adsorbed components was by adsorbed phase surface flow through the pores of the carbon plug. Furthermore, the blockage of gas-phase (Knudsen) diffusion through the pores of the carbon plug by the adsorbed phase created on the surface of the non-porous carbon particles was found to be an effective secondary mechanism in separating gas mixtures by this membrane. In experiments with a mixture of hydrogen and sulfur dioxide, the porous carbon plug became impermeable to the non-adsorbed H.sub.2 while being permeable to the strongly adsorbed SO.sub.2. Very effective separation by this mechanism was also observed with nitrogen-carbon dioxide and neon-carbon dioxide mixtures. In mixtures of argon and nitrogen, which are adsorbed within pores of the plug to a comparable extent, a much lower degree of separation was observed.
In an article entitled "Surface Flow and Separation in Microporous Media", published in the A.I.Ch.E-I.Chem.E. Symposium Series No. 1, pp. 112-121 (1965), R. M. Barrer described experiments on the separation of binary gas mixtures by surface flow in porous glass, carbon, and aluminasilica catalyst plugs having mean pore hydraulic radii between 3.0 and 14.4 Angstroms. It was found that even in the Henry's law range where the adsorption isotherms are linear with pressure, surface flow or surface diffusion through the highest surface area porous carbon plug was the dominant mechanism for gas transport. For a very dense adsorbed phase in the pores, the concomitant gas-phase flow was completely blocked. Separations in binary mixtures of H.sub.2 and SO.sub.2 were studied at -20.7.degree. and -33.degree. C. for the carbon plug or membrane, and highly efficient separation was observed between the adsorbable component (SO.sub.2) and the less-adsorbable component (H.sub.2). Effective separation was also observed between Ne and CO.sub.2, and N.sub.2 and CO.sub.2, at -83.degree. C. A useful enrichment of Ar in Ar-N.sub.2 mixtures at -195.degree. C. was also observed.
R. Ash, R. M. Barrer, and R. T. Lowson studied the diffusion of gases and gas mixtures through carbon plugs and reported the results in a paper entitled "Transport of Single Gases and of Binary Gas Mixtures in a Microporous Carbon Membrane" published in JCS Faraday Transactions I, 69, 2166 (1973). The carbon membrane or plug was prepared by compressing non-porous carbon black into a porous plug 0.976 cm thick having pores with a mean hydraulic radius of 5.5 Angstroms. Binary mixtures of H.sub.2 and N.sub.2, He and NH.sub.3, H.sub.2 and NH.sub.3, and N.sub.2 and NH.sub.3 were tested at 223.degree. to 273.degree. K. with this carbon plug, and the experiments showed that ammonia could be separated from these mixtures very effectively by surface flow in which ammonia adsorbed within and diffused through the pores while blocking the diffusion of the lighter gases. Similar studies using carbon plugs were carried out using helium, carbon dioxide, and various hydrocarbons by R. Ash, R. M. Barrer, and P. Sharma and were reported in an article entitled "Sorption and Flow of Carbon Dioxide and Some Hydrocarbons in a Microporous Carbon Membrane" published in Journal of Membrane Science, 1 (1976) 17-32. Blockage of pores between the non-porous carbon particles in these carbon plugs by adsorbed films of carbon dioxide or hydrocarbons hindered diffusion of helium through the pores, and indicated that this carbon plug can effect separation by surface flow of mixtures of strongly adsorbed and weakly adsorbed components.